Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 345 PM EDT Thu Mar 13 2025 Valid 00Z Fri Mar 14 2025 - 00Z Mon Mar 17 2025 ...The West... Days 1-3... ...Extremely active weather pattern will spread heavy snow across most of the West through Friday, with a second wave approaching for the weekend... A series of potent upper level shortwave troughs will move across the West through this period, resulting in multiple rounds of heavy snow across much of the terrain from the Cascades, Sierras, the Front Range, and much of the terrain in between. This very active period with multiple rounds of snow and low snow levels could result in impacts to the various mountain passes across the West, with snow accumulations measured in feet. The first and most potent of the upper level troughs is moving across the West right now. A vorticity maximum rounding the base of the trough along the West Coast now will move into the Southwest tonight. Ahead of this trough, lee cyclogenesis is ongoing across the northern and central Plains. The combination of this trough and surface lows will focus the heaviest associated snow from the Bighorns of MT/WY south through the Front Range and into the Sacramento Mountains of NM tonight. Snow levels will be above 7,000 ft across these easternmost ranges of the Rockies to start the night, but as the trough and associated cold air moves in tonight, they will drop to generally between 2,000 and 3,000 ft, allowing snow to begin to accumulate to pass level by the start of the day Friday. As the Plains low consolidates into one primary low over western Kansas and Nebraska, the snow should similarly focus across Wyoming, Colorado, and eastern Utah Friday morning, where the heaviest snow totals are expected for the Day 1/Thursday Night & Friday period. WPC 24 hour probabilities through Friday afternoon of 6 inches of snow or more are as high as 80% in southwest Colorado, but are above 50% across much of western Colorado, the Uintas of Utah, and the eastern Mogollon Rim of central Arizona. The low shifts into the eastern Plains by Friday afternoon, ending the snow for the Rockies for this system. A second upper level shortwave and the leading edge of a potent northwesterly jet streak will follow behind the first into the West Coast early Friday morning. The rapid succession of the waves could mean some of Rockies may have only a brief break in the snow, particularly across the Four Corners region, before precipitation rates increase once again associated with the approach of the second wave. This shortwave will be weaker than the first, but will still be potent. The heaviest mountain snow will be across northern California and southwest Oregon during the day Friday. With the influx of Pacific moisture, snow levels will rise to between 3,000 and 4,000 ft in this region, which will still be plenty low enough to impact the passes, especially when adding dynamic cooling from any heavy snow on the windward side of the mountains. 24 hour WPC probabilities for 6 inches or more through Friday are above 50% from the southern Oregon Cascades through the Sierras, with the Klamath Mountains of northern California over 80%. The most potent upper level energy with this wave will track across the Southwest Friday night, then rapidly translate eastward to support the impressive cyclone across the northern Plains and upper Midwest. This will result in a rapid collapse/weakening of any organized precipitation across the Southwest during the day Saturday. A smaller portion of the energy from the original second wave will retrograde towards the Pacific Northwest and merge with a third shortwave moving into the coast. This will keep valley rain and mountain snow ongoing across much of Washington State, Oregon, and Idaho Friday night into Saturday. The Cascades will be the focus for the heaviest snow, with 24 hour WPC probabilities for 6 inches or more of snow above 50% for all of the Cascades, and up to 80% for much of the Oregon Cascades. On Day 3/Saturday Night & Sunday, a fourth trough will approach the Pacific Northwest in the form of a vertically stacked low. That low will approach the coast of Washington and Oregon during the day Sunday. Lingering lift from the previous wave will keep the mountain snow continuing through Saturday night and intensify during the day Sunday, especially across Oregon. It's here that the low will eventually come ashore Sunday night. The greatest lift and moisture flux will be during the day Sunday into Oregon ahead of the low, so the heaviest precipitation associated with it will be then. Since the snow is unlikely to stop at all this weekend into the Cascades, expect the highest elevations to pick up several feet of snow through the period. The Sawtooths and Bitterroots of Idaho and Montana as well as the Yellowstone N.P. area will also pick up several feet of new snow through the weekend as the waves that impact the coast earlier move inland across the Pacific Northwest through the weekend. Wegman ...Northern Plains... Day 2... ...Rapidly deepening low and a transition from rain to a wintry mix and eventually snow is expected to combine with very strong winds to lead to possible blizzard conditions across parts of the eastern Dakotas and western Minnesota. See Key Messages linked at the bottom of the discussion for more information... The southern stream closed low emerging from the Four Corners and into the Southern Plains Friday morning will become a formidable winter storm across the Plains Friday into Saturday. As this closed low emerges into the Plains early Friday, it will continue to deepen, featuring 500-700mb height anomalies by 00Z Saturday that are below the minimum percentile within the CFSR climatology between 850-500mb across a large portion of the Plains. This is an exceptionally deep system, and the combination of the intense height falls with the LFQ of a subtropical jet streak rotating through the trough will result in rapid lee-cyclogenesis beginning as early as Friday morning. This surface low will deepen rapidly, approaching March record pressure levels across the Plains as it tracks from eastern CO Friday morning to eastern SD Friday evening, and then the U.P. of MI Saturday aftn. While there is some uncertainty in the exact placement and track of this low, especially with the potential for numerous low-level centers to rotate around the broader area of low pressure, the overall spread is relatively small. This leads to high confidence in the storm's evolution and area most likely to experience a brief period of heavy snow. With this low deepening rapidly, significant moisture will be drawn northward from the Gulf on impressive isentropic ascent, and the resultant theta-e ridge is progged to lift into a robust TROWAL which will pivot across WI and into MN/ND/SD Saturday morning. As this occurs, the combination of increasing ascent, aided by a pivoting deformation axis W/NW of the low center, and cooling of the column through both dynamic affects and ageostrophic flow/cold air drainage from the NNW, will result in a p-type transition from rain to mixed to snow. There remains uncertainty into how long this transition will take to occur, but the intense omega into the column Saturday morning suggests the guidance may be under-doing the speed at which this occurs, indicating a faster transition from rain to snow, with less freezing rain/sleet in between. This is more typical of the conceptual model for this type of system as well, so while mixed precip is likely at times, the more impactful wintry precipitation type should be snow, which will likely come down heavy at times (1-2"/hr rates). At the same time, very strong winds, which are progged by model soundings to exceed 50 kts in the 800-900mb layer, will help to fracture dendrites leading to low SLRs which will be blow around considerably. Additionally, even with snow coming down heavily it may struggle to accumulate for the first hour or so given warm antecedent ground conditions and prior rainfall. So, while total snowfall may be somewhat modest except beneath the pivot point of this deformation axis, impacts will be substantial with near blizzard conditions possible from far northeast Nebraska through northern Minnesota. In this area, 24-hr WPC probabilities from 00Z Sat to 00Z Sun reach as high as 50-70% for 4+ inches, and isolated totals above 6 inches are possible (20-40%). Additionally, some light freezing rain has a 10-30% chance of accumulating 0.1" or more in a small portion of NW MN. Snell/Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png